Process for liquefaction of natural gas and transportation by marine vessel



Sept. 10, 1968 v, c. w L s ETAL 3,400,547

PROCESS FOR LIQUEFACTION OF NATURAL GAS AND TRANSPORTATION BY MARINEVESSEL Filed July 20, 1967 6 Sheets-Sheet 2 I E 5 I 220 I80 00 -60 -20 O20 60 I09 TEMP F TEMP. "F

60 I00 1/0 I30 140 I P ps in.

mu/n/rozs: wee/1. C. w/LLmMs, OMAR H.5/HOND5,

Sept. 10, 1968 v, c. WILLIAMS ETAL 3,400,547 PROCESS FOR LIQUEFACTION OFNATURAL GAS AND TRANSPORTATION BY MARINE VESSEL Filed July 20, 1967 6Sheets-Sheet 5 F/ELD SITE METHANE psz'a.

IN CYc LE ATTORNEYS Sept. 10. 1968 v. c. WILLIAMS E AL 3,400,547 PROCESSFOR LIQUEFACTION OF NATURAL GAS AND TRANSPORTATION BY MARINE VESSELFiled July 20, 1967 6 Sheets-Sheet 6 llb C soa /fa 1am- L766 150 MAKE-UP162 Paws? v 7 w= 67 w=l06 N: 1360 flfi' V k r MARKET SITE PRocEss x Z NM DIAGRAM 405 L z IEPSIA +05/-' 174176175 2-8? '60 i: 5 {80 V llb cu1595M sat Zz 258F 7 /.05 26 IV; Sat ziq- 32oF I5'PSIA MARKET SITE I-IEATEXCHANGE/Q5 7 32.12.

/80 I45 -I00 60 2o m 200 EACH TANK Co/vmuvs [MIT/ALLY 0.00525 Cu FT 4 H2 /LIQ NITROGEN 0.262 LB LIQ NITROGEN EAo-I TANK HALF FULL MAREEI FIELD1.05 1.6 NITROGEN ILB NITROGEN EACH TANK CONTAINS INITIAL-LYO-OI CU FT 23 5. :/LIQ METHANE, 0-262 L8 L/Q METHANE 4 H TANK: FULL INVENTORs:

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.. OMAR {-1.5IMON05,

FIELD AgKET 5v/.74L/4,E?M azuwmw I 1.6 METHANE 202 1-05 LB METHANEHTTOPNEYS United States Patent .s

3,400,547 PROCESS FOR LIQUEFACTION OF NATURAL GAS AND TRANSPORTATION BYMARINE VESSEL Virgil C. Williams, 104 Frontenac Forest, St. Louis, Mo.63131, and Omar H. Simonds, Essex Fells, N.J.; said Simonds assignor ofthirty-five percent to said Williams Continuation-impart of applicationSer. No. 591,496, Nov. 2, 1966. This application July 20, 1967, Ser. No.654,935

23 Claims. (Cl. 62-55) ABSTRACT OF THE DISCLOSURE Natural gasliquefaction and transportation from a field site to a market site isaccomplished by transporting liquid nitrogen or liquid air by ship fromthe market place to the field site where it is used to liquefy thenatural gas. The availability of the cold or refrigeration in the liquidnitrogen or air reduces the equipment required at the field site. Liquidnatural gas is pumped into the holds of the same cryogenic tanker shipused to transport the liquid nitrogen or air and the liquefied naturalgas is returned to the market site. Regasification of the liquid naturalgas is carried out to use the cold or refrigerant effect therein toliquefy nitrogen or air, which is then charged to the returning tankerto the field site. The process is a repetition of liquid nitrogen orliquid air. from the market site to the field site and there used toliquefy natural gas, which is then shipped from the field to the marketwhere the liquid natural gas is gasified and in the process liquefiesnitrogen or air for return to the field. The cycle employs about 1.0 to2.1 pounds of nitrogen to 1.05 pounds of methane. In the process of boththe field site and the market site, various cycles employing expansionof the various high pressure streams for cooling is employed and thework is matched against compression of other streams and subsequentexpansion for efficiency in the system and minimization of powerrequirement.

CROSS REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of our application Ser. No. 591,496, filed Nov. 2,1966 and now abandoned.

BACKGROUND OF THE INVENTION There is an economic use for the greatquantities of unused natural gas in areas remote from the worldsmarkets. There are two classes of natural gas: (a) gas produced from gaswells and (b) natural gas produced in association with crude oil. Muchof this gas production occurs in vast fields in the developing countriesand cannot be transported economically by pipepline to these worldmarkets. Because of this, only a fraction of the worlds natural gasresources are at present usefully consumed.

It is one thing to flare or release natural gas at great pressure, it isanother to reduce it to manageable proportions, transport it, store it,and sell it in competition with other fuels.

One way to deal with a large volume of gas is to liquefy it. However,natural gas, which is largely methane, cannot be liquefied by simplyincreasing the pressure, as has been the case with heavier hydrocarbonsused for energy purposes. The critical temperature of methane is -1l6.5R, which is the temperature above which it is impossible to liquefymethane regardless of the pressure applied. At atmospheric pressure, theliquefied methane will be at its normal boiling point of -258 F. Hence,the techniques of liquefying and handling natural gas are within thescope of the field of cryogenic technology.

3,400,547, Patented Sept. 10, 1968 The liquefaction of natural gasrequires the removal of energy in the form of sensible and latent heat.This process can be accomplished by mechanical refrigeration where heatis transferred by a series of refrigerants to a reject ambienttemperature-level heat sink. This method uses what is referred to as acascade cycle or process.

Another method is that of compressing and expanding the gas, usingturbine-expanders. This is known as an expander cycle.

The most widely used cycle is the cascaded vapor compression system.This cycle, based on refrigeration principles, uses commerciallyavailable refrigerants whose thermodynamic and physical properties arefairly wellknown. Storage of large volumes of natural gas is mosteconomical in its liquefied form. As a liquid, natural gas will occupyapproximately of its gaseous volume under standard conditions (625 cu.ft. of gas at standard temperature and pressure=1 cu. ft. liquid atnormal boiling point). Two areas of interests show need for storage ofliquefied natural gas.

The first area of interest lies in transporting gas from energy-rich toenergy-poor areas in insulated tankers, whereby storage depots at boththe field site and the market site are required. In such base-loadoperations, the storage facility is essentially a surge tank smoothingout the nonuniform base-load demand and fuel delivery operations. Inthis case, it is not necessary to preserve the liquefied natural gas inthe tank for long periods, since it is passed on to distribution withina short period of time.

The second area of interest is in the continued growth and expansion ofthe natural gas industry which has resulted in the need for storage oflarge volumes of gas near metropolitan areas to meet winter peak-loads.In this case, liquefied natural gas is stored for relatively longperiods of time and used during only a few days of the winter.Consequently, heat influx must be held to a minimum.

A great deal of research is at present being devoted to finding andrefining industrial processes which will make continuous use of the verylarge amounts of cold available. The main possibilities here lie in thedevelopment of major industrial applications such as the production byair rectification of liquid oxygen, nitrogen, etc.

Ideally, to make the maximum use of cold potential, it is necessary thatsuch use should show a considerable reduction in capital investment andoperating costs compared with conventional processes for making somesaleable commodity.

The cold should replace low-temperature refrigeration which is veryexpensive compared to mild refrigeration (e.g. the low temperaturesneeded in the production of cryogenic liquids). The overall cost ofrefrigeration in the liquefaction of the natural gas is a major part ofthe final sales price.

There is, however, another manner of approaching the use of theavailable cold. This would be to consider the cold as an inherent partof the process thus making it the commodity; not selling it or somethingderived from it, but building its use into the process technology toreduce the investment and operating costs and thus' to increase theprofitability of a natural gas distribution venture.

SUMMARY OF THE INVENTION By means of this invention, a process has beendevised for liquefaction of nitrogen at the market site against theevaporating liquefied natural gas and returning the liquid nitrogen inthe insulated transport tanks in the marine vessel to the field sitewhere it is employed in liquefaction of the natural gas, which is thenloaded on the marine vessel for shipment to the market. The availabilityof the cold in the liquid nitrogen reduces greatly the equipmentrequired at the field site. The cycle is a constant repetition of thesetwo factors, namely, shipping liquid gas to the market and returningliquid nitrogen back to the field, and using the refrigeration effect atboth the field and the market sites of the liquefied gas shipped to thesite to liquefy the gas shipped from the site. Nitrogen may be separatedfrom air at the market site by air rectification, thus producing oxygengas, which at the market, is of great use in the chemical andmetallurgical industries. No auxiliary liquefaction means are needed atthe field site and all of the energy necessary for liquefaction of thenatural gas is provided through the liquid nitrogen shipped from themarket site to the field site.

It is also possible to employ liquid air because of its major proportionof nitrogen instead of liquid nitrogen in this cycle for shipment to thefield site. Air, because of its high percentage of nitrogen, hassubstantially similar thermal characteristics to nitrogen. In this casethe liquid storage tanks would require purging with nitrogen to removethe oxygen containing atmosphere to avoid possible explosive mixtures ofnatural gas and air. This purged nitrogen is obtainable from anauxiliary supply or from fractionation of the liquid air either at thefield site or the market site.

The large refrigeration complexes required to liquefy the natural gas atthe field site are thus obviated, and economy-wise the capital equipmentlocated in the foreign field site is reduced to a very minimum. Theprocess is initiated for each ship by sending outbound from market siteto field site one cargo of liquid nitrogen which can be accumulated byany economical means in the market site.

The above features are objects of this invention and further objectswill appear in the detailed description which follows, and will beotherwise apparent to those skilled in the art.

For the purpose of illustration, there are shown in the accompanyingdrawings examples of the process of this invention. It is to beunderstood that these drawings are for the purpose of example only andthat the invention is not limited thereto.

In the drawings:

FIGURE 1 is a flow sheet showing a typical field site process forliquefaction of natural gas;

FIGURE 2 is a flow sheet showing a typical market site process for theliquefaction of nitrogen;

FIGURE 3 is a graph showing the heat transfer in the field siteexchanger;

FIGURE 4 is a graph showing the heat transfer in the market site heatexchanger;

FIGURE 5 is a graph showing a typical pressure relationship at the fieldsite for the proportion of nitrogen to methane needed for liquefaction;

FIGURE 6 is a graph showing the vapor pressure curves for nitrogen andmethane;

FIGURE 7 is a flow sheet of a modified process employing higherpressures and showing a typical field site process for liquefaction ofnatural gas;

FIGURE 8 is a block diagram showing the arrangement of the equipmentemployed in the fiow sheet of FIGURE 7;

FIGURE 9 is a graph illustrating the flow of heat in the field siteexchangers;

FIGURE 10 is a flow sheet of the modified example employing higherpressures and showing a typical market site process;

FIGURE 11 is a block diagram showing the arrangement of the equipmentfor the market site process of FIGURE 10;

FIGURE 12 is a graph showing the flow of heat in the market site heatexchangers of FIGURE 10; and

FIGURE 13 is a schematic view of the tankers employed in the transfer ofliquid nitrogen from the market site to the field site and the returntanker shipment transferring liquefied natural gas from the field siteto the market site.

DISCLOSURE Natural gas, which is comprised principally of methane withsmall percentages of ethane, propane, butane, and minimal percentages ofcarbon dioxide and nitrogen, is available in many places in the world,but in most cases the markets for this gas are in industrial countriesremote from the field sites. In many situations pipe lines are used totransport the gas from the source to the market. However, there arecircumstances where pipe line transportation is technically andeconomically unfeasible. Such cases are, for example, the transportationof Arabian or Algerian gas to England or Germany, and Venezuelan orMexican gas to Florida or the each coast of the United States, or theNorth Sea area. It has been proposed to liquefy natural gas at the fieldsite and take it by barge or ship to the market site. At the market siteit would be re-gasified and pumped into the distribution system.

In the instant invention the great refrigeration available in the liquidnatural gas is employed to liquefy nitrogen at the market site byevaporating the natural gas in heat exchange relation with the nitrogen.The liquid nitrogen is then returned to the transport tanks in insulatedmarine vessels, although it will be understood that other insulatedcarriers, such as trucks, railroad tank cars and the like, may beemployed, for transport to the field site. At the field site theliquefied nitrogen returned in the tanker, or other vessel, is employedin liquefying the natural gas, which is then returned in the tanker tothe market. By this invention the marine vessel is not deadheaded whenit is returned from the market site back to the field site, and thecycle is a constant repetition of taking the liquid natural gas tomarket and then the liquid nitrogen back to the field.

Nitrogen is separated from air at the market site, such as by airrectification, and oxygen gas is produced as a byproduct, which is ofgreat use in the chemical and metallurgical industries. The oxygen thenbecomes a valuable by-product and a premium factor for the cycle of thisinvention.

In considering the illustration of the invention, it will be assumedthat an insulated marine tanker is employed, although as above statedother insulated carriers, such as railroad tank cars, truck carriers,and the like, may be used.

Example 1 The unit basis chosen for illustration of this invention isthe receipt at the market site of one pound of methane, which is themajor constituent of natural gas and for practical purposes isconsidered in calculations demonstrating this process and the two termsmay be used interchangeably. In the air rectification at the market sitethe input may be, as an example, 2.775 pounds of air, which will produce2.1 pounds of nitrogen and 0.675 pound of purity oxygen. The market siteprocess for nitrogen liquefaction obtained through heat exchangerelation with the methane by evaporation from the liquefied gas iscarried out on the basis of the use of one pound of methane to 1.0-2.1pounds of nitrogen. The methane vaporizes and moves or is compressedinto the market pipe lines for distribution. The liquid nitrogenproduced is put into the insulated storage tanks of the tanker and istaken back to the field site. In this transportation about five percentof the nitrogen is lost by evaporation, thereby delivering about 95% ofthe nitrogen liquefied at the market site to the field site.

The field site cycle evaporates the liquefied nitrogen againstcondensing methane and the liquid methane is then put into the insulatedstorage tanks of the tanker. This cycle uses about 1.0 to 2.1 pounds ofnitrogen to 1.05 pounds of methane. The lower ratio, i.e., 1 pound ofnitrogen to 1 pound of methane represents the optimum for shipconstruction but requires a greater pressure of methane employed at thefield site. In the limit for tank filling storage capacity on theshipping vessel, about 2 pounds of nitrogen can be carried per 1.0 poundof methane. As the pressure is reduced at the field site, representingeconomies in compression, the ratio of nitrogento methane is increasedwhich deviates from optimum values for ship construction. FIGURE 5 showsa typical curve for the cycles shown in FIGURES 1 and 2 for the effectof pressure on the nitrogenzmethane relationship. The methane on beingreturned to the market in the tanker loses about 5% by evaporation, so1.0 pound is delivered to the market site Where it is evaporated in heatexchange relation with nitrogen to produce liquid nitrogen for shipmentof the liquefied nitrogen in the same tanker. back to the field. Theprogram and process consists of repetition of these basic cycles. Someintermediate storage may be employed at both the market and the fieldsites for transfer purposes through insulated tanks employed to preventloss of refrigeration.

In the field site flow sheet of FIGURE 1 and the market site flow sheetof FIGURE 2, the cycles employed at these two sites are disclosed. Inthe field site it will be noted that the features employed use liquefiednitrogen in heat exchange relationship with the natural gas, followed byexpansion and consequent cooling of the nitrogen, after which thenitrogen is again used in heat exchange relation with the natural gas toobtain the fullest possible effect of refrigeration available. Thenitrogen may be vented to the atmosphere upon the completion of thefield site process or used in any other fashion desired. In the marketsite process the fullest possible effect of refrigeration from theliquefied natural gas is employed by sending compressed nitrogen in heatexchange relation with the liquefied naturalgas and then expanding amajor portion of this to reduce the pressure and cool the nitrogen toliquefaction. A portion of the nitrogen stream is drawn off and used inheat exchange relation with the nitrogen stream before expansion andthen compressed and fed back into the introduction line for the nitrogenbefore it is passed into heat exchange relation with the liquefiednatural gas. In this manner a highly eificient use of refrigeration ismade available to liquefy the nitrogen.

At the field site, the liquid nitrogen is pumped out of the storagetanks of the tanker, or other carrier, and is replaced with liquidmethane.It is a significant phenomenon of the two substances, nitrogenand methane, in the process of this invention, that the quantitiesdemanded for flow in the heat exchange stages on a mass basis for heatbalance are also in acceptable balance on a liquid volume basis.

The design of the various elements in the cycles of this invention is inaccordance with the thermodynamic properties of the individualmaterials. The data given in the examples are dealt with graphically tosatisfy the requirements of the first and second laws of thermodynamicsand also to have real significant temperature differences that permit ofeconomic design of the heat exchangers.

FIELD SITE PROCESS The field site process of Example 1 is shown in FIG-URE 1 and graphically shown for the field site exchanger heat flowvalues in FIGURE 3.,In the field site process, liquefied nitrogen atatmospheric pressure is returned by the insulated tanker from the marketsite to the field site, and is pumped to working pressure by a liquidpump. The cold nitrogen heat exchanges countercurrently against warmmethane gas, which thus is brought to a condensed liquid state atatmospheric pressure. This liquid methane is then put into the tankerfor shipping to the market site. To make one pound of methane availableat the market site, 1.05 pounds is shipped from the field and 0.05 poundis lost through vaporization or may be vented or used as fuel in anengine or other purposes while in transit.

In the process of FIGURE 1, liquid nitrogen is charged to anintermediate insulated storage tank 10. From the insulated tank 10, 1.7pounds of liquefied nitrogen is charged at 320 F. and atmosphericpressure, i.e., 14.7 p.s.i.a. to pump 18 where it is compressed to 800p.s.i.a., for purpose of example, to 310 F. The still liquid nitrogen isthen charged through line 20 to heat exchanger 22, where it passes incountercurrent relationship to natural gas. The nitrogen passes throughthe heat exchanger in pass 24 and leaves at 55 F. and about 800 p.s.i.a.where it is expanded through a turbine expander 26 to p.s.i.a. and l80F. The nitrogen under these conditions is charged through line 28 topass 29 in the heat exchanger, and in the process is heated at at 50p.s.i.a. and is vented in line 30. The natural gas from the field sitein the amount of 1.05 pounds at 80 F. and at 800 p.s.i.a. is chargedinto the heat exchanger in line 32. It goes through the heat exchangerin pass 34 and leaves as liquefied natural gas at atmospheric pressureat 260 F. The thus liquefied natural gas is charged in line 36 toinsulated storage tank 12 and from it can be charged to the carrier suchas a marine tanker or the like. In the process, the turbine expanderdelivers 99 B.t.u. of work, which must be absorbed and the heatexchanger transfers 385 B.t.u. In many cases the field site natural gaspressure is below 800 p.s.i.a. and the turbine work may be convenientlyused in a compressor to raise the natural gas pressure to 800 p.s.i.a.As an example of alternate pressures and quantities of nitrogen employedin the process, 1.4 pounds of nitrogen may be employed at 1500 p.s.i.a.,in which case the turbine expanders will deliver 89 B.t.u. of work andthe heat exchanger will transfer 375 B.t.u.

MARKET SITE PROCESS The market site process of Example 1 is shownschematically in FIGURE 2, while FIGURE 4 shows the heat transferbetween the methane and the nitrogen by way of a graph. In the marketsite process, 1 pound of liquefied natural gas is taken from the carrierin line 40 and charged to the intermediate tank 14. One pound ofliquefied natural gas is taken from the storage tank through line 42 andcharged to the heat exchanger 44 at 260 F. and atmospheric pressure. Theliquefied natural gas passes through pass 46 in the heat exchanger andin heat exchange relation with the nitrogen leaves in line 47 at F.where it can be passed to distribution systems or storage tanks or thelike for ultimate use and sale at the market site. The nitrogen employedin this system can be obtained from air rectification, as will be wellunderstood in the art, and this process leaves available oxygen forindustrial purposes. Nitrogen in the amount of 1.78 pounds is employed,since this provides 1.7 pounds at the field site since there is 5% lossof nitrogen in shipping back to the field through evaporation or thelike. This nitrogen is charged through the line 48 at F. and at 250p.s.i.a. through pass 50 in the heat exchanger, where it is cooled to255 F. To this is added .725 pound of a compressed flash stream ofnitrogen, so that in line 52, 2.505 pounds of nitrogen are obtained. Thenitrogen is cooled and condensed to liquid at 255 F. while the methanevaporizes at 260 F. and superheats to +60 F. at 15 p.s.i.a.

The liquid nitrogen in line 52 is subcooled from 255 F. to -310 F.against 0.725 pound of flash gas which warms from 320 F. to 260 F. inline 54. This process is effected in heat exchanger 56. The flash gasslip stream is then compressed from 15 p.s.i.a. at260 F. in compressor58 to 250 p.s.i.a. at 80 F., and is added in line 60 to the feed streamof 1.78 pounds of nitrogen, which originally enters the process throughline 48. The pull off from the separator bottle 62 in line 64 provides1.78 pounds of liquid nitrogen at atmospheric pressure and attemperature of 320 F. The liquefied nitrogen is charged from line 64 tothe intermediate storage tank 16 or directly to the insulated marinetanker or other insulated carrier as provided in this process. Thecompression work for the slip stream gas is 58 B.t.u.

FIGURE 6 is a vapor pressure curve listing the vapor pressures formethane and nitrogen.

To summarize the example process, and to provide for the loss intransit, 1.05 pounds of liquefied natural gas is charged 'at the fieldsite to the tanker to provide 1.0 pound of liquefied natural gas at themarket site. At the market site 1.78 pounds of nitrogen is provided -forshipment in the tanker to provide 1.70 pounds of liquefied nitrogen atthe field site.

Example 2 In this example a high pressure system is employed for thefield site process and market site process initially described inExample 1. Utilization is made in the full of work provided for theexpansion of the various gases to be used in compressing other streamsto utilize full efiiciency and economies in the process. The field siteprocess is disclosed in FIGURES 7, 8 and 9, while the market siteprocess is disclosed in FIGURES 10, 11 and 12.

FIELD SITE PROCESS The field site process of Example 2 is shownschematically in FIGURE 7 and in block diagram in FIG- URE 8. The graphof FIGURE 9 shows the heat transfer between the methane and nitrogen. Inthis process one pound of liquid nitrogen is charged at 320 F. andatmospheric pressure, which, for purpose of illustration, is taken at 15p.s.i.a. in line 100 to a pump 102. In the pump the liquid nitrogen ispressured to 2000 p.s.i.a. at -300 F. The shaft work is provided through'a turbine expander (impulse or reaction type) 104 in the methane streamas will be further described. From the pump, the nitrogen is chargedthrough heat exchanger 106 in counter-current relation with methanestream heat exchanger 108. The nitrogen refrigerant is then chargedthrough heat exchanger 110, which along with heat exchanger 112, 114 and116, all used in the nitrogen refrigerant stream as will be laterdescribed, are in heat exchange relationship with the methane heatexchanger 118. From the heat exchanger 110 the nitrogen at 'atemperature of 0 F. passes through a turbine expander 120 to a reducedpressure of 300 p.s.i.a. and a reduced temperature of 190 F. The turbineexpander 120 is matched with a compressor 122 which is employed tocompress the methane in the feed stream as will be later described. Thenitrogen refrigerant from the turbine expander 120 then is passedthrough heat exchanger 112 in further heat exchange relation with themethane heat exchanger 118 from which it is then introduced to anotherturbine expander 124 where the pressure is reduced to 30 p.s.i.a and thetemperature is reduced to about 190 F. The turbo expander 124 is matchedwith a compressor 126 in the nitrogen stream, which later compresses thedownstream nitrogen as will be further described.

From the turbine expander 124, the nitrogen stream is introduced intoheat exchanger 114 in heat exchange relation with the methane heatexchanger 118 in the same fashion as the previous streams. From the heatexchanger 114, the nitrogen refrigerant is introduced to the compressor126 where it is compressed and then cooled in aftercooler 128. Anadditional compressor 130 is also employed to further increase thepressure and work is provided through turbine expander 132, which willbe further described in the nitrogen stream. From the compressor 130,further cooling is effected in cooling unit 133 after which the nitrogenstream is passed through heat exchanger 134 to obtain a temperature ofF. The nitrogen is then introduced at 200 p.s.i.a. to the turbineexpander 132 where it is expanded to 18 p.s.i.a. and 190 F. Thisnitrogen is then passed through the last heat exchanger 116 in heatexchange relation with the methane heat exchanger 118. The nitrogenrefrigerant leaving heat exchanger 116 then passes through heatexchanger 136, and is exhausted through line 138 for any eventualdesired usage at 18 p.s.i.a. and F.

The incoming methane stream is introduced into the field site processthrough line 140 at 800 p.s.i.a. and in an amount for the process shownof 1.05 pounds methane. The methane is further compressed in thecompressor 122 with work being provided through the nitrogen turbineexpander 120, as previously described. Auxiliary refrigeration isprovided through cooler 142 and the methane is then introduced at 10 F.and at a pressure of 1500 p.s.i.a. into the methane heat exchanger 118.After leaving the heat exchanger 118, with the consequent cooling by thefour nitrogen refrigerant heat exchange units 110, 112, 114 and 116, themethane is passed through heat exchanger 108 for a final phase of heatexchanger cooling in heat exchange relation with the methane heatexchanger 106. The methane is then passed through the turbine expander104 for further reduction in pressure to 15 p.s.i.a. and a temperatureof -258 F. to provide the 1.05 pounds of methane in liquefied form. Thework provided in the turbine expander 104 is used to drive the pump 102,as previously described.

FIGURE 8 shows the field site process in block diagram with a matchingof the compressors and turbine expanders in simplified form. It furthershows the heat transfer in the various heat exchange units and the workprovided in the matched turbine expanders and compressor pairs. Thegraph of FIGURE 9 illustrates the following of the second law ofthermodynamics in the field site process by having real temperaturedifferences with the refrigerant nitrogen stream always at a lowertemperature than the methane stream from which the nitrogen isabstracting heat The compressors employed may be diaphragm cooledmachines with a ratio of isothermal to isentropic ideal works of 0.748but corrected to actual work by the multiplier 1.175. The isentropicwork is calculated at 83% efficiency.

MARKET SITE PROCESS The market site process of Example 2 is shown inFIGURES 10, 11 and 12. FIGURE 10 is a schematic diagram and flow sheetof the nitrogen refrigerant and methane streams, while FIGURE 11 is ablock diagram illustrating the work output and input matching of theturbine expanders and compressors to provide for efficiency and economyin the process. FIGURE 12 is a graph showing the heat transfer betweenthe nitrogen and methane to provide a real temperature differencebetween these streams and illustrates the colder temperature of themethane for abstraction of heat from the nirogen stream. The figures inthe market site process are based on 1.05 of nitrogen feed at 1atmosphere pressure and 85 F. against 1.0 pound of methane in liquefiedform, which is being vaporized and processed.

At the market site the liquid methane is vaporized and delivered intothe pipe line grid at 600 p.s.i.a. The heat required for thisvaporization is used to condense nitrogen gas or (liquid air) which isthen returned to the field as a source of refrigeration to liquefy thenatural gas. Of the two cycles, the field site and the market site, thelatter is the more difiicult to analyze. There are several workablecycles, among them being mixed gas (nitrogen and methane) heat pumping,straight heat pumping (as shown in this example), nitrogen heat pumping,all of which are novel in the invention as described herein. With airemployed there is also the possibility for fractionation Within thecycle to deliver liquid nitrogen and oxygen gas for an added economicbenefit. The amount of oxygen in a plant size installation would bequite considerable and could support a satellite chemical ormetallurgical enterprise.

In the process as shown in FIGURES l0 and 11, 1.05 pound of methane insaturated liquid form at 15 p.s.i. and -258 F. is introduced in theprocess through line where it is split into a first stream in line 152and a second stream in' line 154. The first stream in the amount of0.385 pound methane is passed through heat exchanger 156, which is inheat exchange relation with nitrogen heat exchangers 158 and 160, aswill be later described. The methane stream, after passing through heatexchanger 156, is at a temperature of 245 F. when it is introduced intocompressor 162 where it is compressed to 600 p.s.i.a. and a temperatureof 110 F. in stream 164. The compressor 162 is matched with a turbineexpander 165,.which 'is provided in the second stream of methane as willbe more clearly described below. This matching provides that the workoutput from the turbo expander 166 is supplied to the compressor 162 forpart of the power required. Stream 164 is combined in final outputstream 166 with a second stream output to provide 1.0 pound methane at600 p.s.i. and 120 F.

The second liquid methane stream in an amount of 0.615 pound methane ispressured by pump 168 to 1500 p.s.i.a. and a'temperature of 245 F. Partof the work required for thepump is provided through turbine e-xpander170 in the nitrogen stream, as will be more clearly described below. Theliquid methane pumped by the pump 168 is introduced through line 172 tomethane heat exchanger 174, which is in heat exchange relation withnitrogen heat exchangers 176 and 178, as will more clearly be describedbelow in the discussion of the nitrogen stream flow. The methane fromheat exchanger 174 is introduced at a temperature of 60 F. tosuperheater exchanger 176 where it is heated to a temperature of 260 F.The exhaust gas from a makeup power source, which may be used for thecompressor 180 in the nitrogen stream, to be later described, may beused as a source of heat in the superheater exchanger. The methane fromthe superheater exchanger is introduced to the turbine expander 165,previously described, for exhaust to 600 p.s.i.a. at a temperature of120 F. where it is then introduced to the final output stream 166 fordistribution to the natural gas system and the ultimate pipeline grid at600 p.s.i.a.

The nitrogen is introduced into the process through line 179 in theamount of 1.05 pounds nitrogen at 15 p.s.i.a. and 85 F. This stream ispassed through heat exchanger 176 where with recycled nitrogen it isintroduced in line 182 to compressor 180. The nitrogen in line 182 is at15 p.s.i.a and 240 F. and is compressed by the compressor to 265p.s.i.a. and a temperature of 85 F. The total stream of the nitrogenintroduced into the system and the recycled nitrogen is in the amount of1.316 pounds nitrogen. This combined stream in line 184 is passed to theheat exchanger 178, and is then passed through heat exchange-rs 160 and186 for further cooling. The nitrogen is then passed through the turbine(impulse or reaction) [170 for reduction in pressure and further coolingand introduced into separator 188. From the separator liquefied nitrogenin the amount of 1.05 pounds is withdrawn in stream 190 at a pressure of15 p.s.i.a. and 320 F. for delivery to the tanker bound for the fieldsite.

A recycle nitrogen stream is taken from the separator 188 in line 192'and passed through recycle heat exchangers 194 and 158 for combinationwith the initial feed nitrogen in line 182, as previously described.This provides further cooling for the process.

FIGURE 12 graphically shows the real temperature differences in thesystem with the refrigerant nitrogen and methane streams employed in theprocess always being colder than the nitrogen stream from which the heatis being abstracted.

In the process the compressors utilized in the cycle may be coldsuctionhigh gas density centrifugal machines. Efficiencies are high in thisequipment, particularly for the axial flow machines and can be taken as0.85 (isentropic). The net power supply to the market site process maybe in the form of a makeup machine which may be a gas turbine for thecompressor 180 with, as previously mentioned, the exhaust gas being usedas a heat source in the superheater exchanger 176. The heat exchangerunit 156, 158 and 160 may be a multipass condenser evaporator. Themaximum pressure of 265 p.s.i.a. used in these units allow the use ofso-called low pressure type exchangers. The heat exchangers 186 and 194used for subcooling the nitrogen may also be of this type, while thehigher pressure heat exchangers 174, 176 and 178 may be of the woundmandrel type.

Heat exchanger 176, the low pressure gaseous nitrogen (or air) heatexchanger may be installed in duplicate form with the matching amount ofheat transfer surface from heat exchanger 174 to act as a water andcarbon dioxide knockout surface for the incoming low pressure nitrogenor air. Thus, when one of the duplicate surfaces is rimed the heatexchanger surface is switched to the other duplicate in the methodcalled switch heat exchangers in the cryogenic industry, while the rimedsurface is heated and derimed preparatory to the next switching. By thismeans an expensive drying system on the feed nitrogen (or air) isobviated.

SHIPPING STAGE The shipment of the nitrogen refrigerant and natural gasis on a 1:1 mass basis. For analysis on the basis of 1 pound of methaneand nitrogen there would be shipped from the market site to the field1.05 pounds liquid nitrogen (or liquid air) in the outbound vessel,which would provide 0.021 cubic foot of liquid. The receipt at the fieldsite, allowing for some evaporation, is considered as 1.00 pound ofliquid nitrogen. In the reverse shipment from the field site to themarket site, 1.05 pounds of liquid methane would be provided on theoutbound vessel, which would constitute 0.040 cubic foot of liquid sincethe density of liquid methane is about one-half of the liquid nitrogen.At the market site there would be received, after allowing forevaporation, about 1.00 pound of liquid methane or natural gas.

In FIGURE 13 the-re is shown a schematic diagram illustrating theaforementioned shipment considerations which may be used in bothExamples 1 and 2. The outbound vessel from the market site to the fieldsite carrying the liquid nitrogen is indicated by the reference numeral200, while the return vessel from the field site to the market sitecarrying the liquefied methane is indicated by the reference numeral202.

After arrival from the market site at the field site with all four holdtanks only one-half full in the vessel 200, hold tank No. 1, containing0.25 pound liquid nitrogen, would be pump transferred to a field siteland storage tank. Hold tank No. 2 liquid nitrogen would be pumped intomethane liquifier process and 0.262 pound of liquid methane,representing one-fourth of the total charge of 1.05 pounds, would be putinto empty hold tank No. No. 1. This process would be repeated for eachhold tank in turn and then finally the 0.25 pound of liquid nitrogen inthe field site land storage tank would go into the methane liquefierprocess to deliver the 0.262 pound liquid methane representing the lastquarter charge of liquefied methane to hold tank No. 4. The ship is thenfilled with the 1.05 pound liquid methane, used as a base for purpose ofexample, for its trip from the field site to the market site.

It is to be noted that the minimal land storage requirement, whichassumes no field site turn around lost time for the liquid nitrogen, is0.005 cubic foot (0.00525/). This is one-eighth the volumetricequivalent of the full liquid methane ship load of four hold tanks at0.01 cubic foot each or 0.04 cubic foot total. If desired, doublestrength tanks could be built for the No. 2 and No. 3 hold tanks in themiddle of the vessel and they could then be filled with liquid nitrogenfor the trip to the field site, while tanks No. 1 and No. 4 would rideempty or deadhead on this trip..

The purging of the vessel or hold storage tanks at both the field siteand the marked site deserves consideration. At the field site a tankwhich contained liquid air over which the equilibrium vapor associatedwith that liquid air is maintained will, when empty of all of thatliquid air, have a vapor of 95% nitrogen content at one atmospherepressure. According to known flammability limits, this 95% nitrogen,which would have oxygen vapor admixed, would with any amount of methanebe nonflammable. Thus, for the field site, no special purging conditionsare required on refilling the tanks with liquid methane when they areemptied of liquid air providing the above conditions are satisfied.

At the market site, the liquid methane after removal from the vesseltanks will leave a hold full of methane vapor. Since the hold isrelatively warm (minus 258 F.) compared to the charge of liquid air(minus 318 F.), the cool down of the hold tank would initially vaporizesome of the liquid air charge and this path would direct itself acrossthe tip of the flammable region. Accordingly, it would be necessary toperform the tank cool down with liquid nitrogen at the market site.

Alternatively, as the liquid methane is pumped from the ship tanks atthe market, the overlaying methane vapor can be displaced with nitrogenvapor to maintain an essentially nitrogen vapor space. So long as thepumped out tank has a methane content of less than 12% in themethane-nitrogen vapor mixture, liquid air can be pumped in directly andhave the tank space in the non-flammable region.

Various changes and modifications may be made within this process aswill be readily apparent to those skilled in the art. As an example,air, which, of course, is principally composed of nitrogen, can be usedin this process instead of nitrogen, and when speaking of nitrogen it isto be understood that other gases such as air in which the majorconstituent is nitrogen can be employed where the thermalcharacteristics of the gas are similar to nitrogen. Such changes andmodifications are within the scope and teaching of this invention asdefined by the claims appended hereto.

What is claimed is:

1. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant which is evaporated to obtainavailable refrigeration effect, said refrigerant being composedprincipally of nitrogen, transferring the liquefied natural gas to aninsulated transport carrier for shipment to the market site,transferring liquefied refrigerant from an insulated transport carrierwhich is also used as a carrier for liquefied natural gas to the fieldsite for use in said aforementioned step of liquefying natural gas atthe field site, said liquefied refrigerant being obtained by evaporatingliquefied natural gas in heat exchange relation with refrigerant gas toliquefy the refrigerant, said liquid refrigerant being compressed beforeit is passed in heat exchange relation with the natural gas and aftersaid heat exchange being expanded to a lower pressure and lowertemperature andpassing the so-cooled refrigerant in a second heatexchange stage into heat exchange relation with the natural gas.

2. The method of claim 1 in which the liquid refrigerant transportedfrom the market site to the field site is in the ratio of about 1.0pound to 2.1 pounds for 1.05 pounds of liquefied natural gas transferredfrom the field site to the market site.

3. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant composed principally ofnitrogen, transferring the liquefied natural gas to an insulatedtransport carrier for shipment to the market site, transferringliquefied refrigerant from an insulated transport carrier, which is alsoused as a carrier for liquefied natural gas to the field site, for usein said aforementioned step of liquefying natural gas at the field site,said liquid refrigerant being compressed before it is passed in heatexchange relation with the natural gas and after said heat exchangebeing expanded to a lower pressure and lower temperature and passing theso-cooled refrigerant in a second heat exchange stage into heat exchangerelation with the natural gas.

4. The method of claim 3 in which the expansion of the refrigerant iscarried out in a turbine expander.

5. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant composed principally ofnitrogen, transferring the liquefied natural gas to an insulatedtransport carrier for shipment to the market site, transferringliquefied refrigerant from an insulated transport carrier, which is alsoused as a carrier for liquefied natural gas, to the field site for usein said aforementioned step of liquefying natural gas at the field site,said refrigerant being compressed atthe market site and then beingpassed into heat exchange relation with liquefied natural gas, saidrefrigerant being further cooled in a main stream by reducing itspressure and being split into a liquefied product stream and a recyclestream.

6. The method of claim 5 in which the recycle stream is passed in heatexchange relation with the main stream of refrigerant before thepressure is reduced and is then compressed and added to the refrigerantgas passed in heat exchange relation with the liquefied natural gas toform the main stream of refrigerant.

7. A method for transportation of natural gas from a field site to amarket site which comprises liquifying compressed natural gas at thefield site by heat exchange with a liquid refrigerant which isevaporated to obtain available refrigeration effect, said refrigerantbeing composed principally of nitrogen, transferring the liquefiednatural gas to an insulated transport carrier for shipment to the marketsite, using the liquefied natural gas to liquefy refrigerant andreturning the liquefied refrigerant to the field site in an insulatedtransport carrier for use in said aforementioned step of liquefyingnatural gas at the field site, said liquid refrigerant being compressedbefore it is passed in heat exchange relation with the natural gas andafter said heat exchange being expanded to a lower pressure and lowertemperature and passing the so-cooled refrigerant in a second heatexchange stage into heat exchange relation with the natural gas.

8. The method of claim 7 in which the liquefied refrigerant is chargedto the carrier at the market site in the ratio of about 1.0 pound to 2.1pounds for 1.05 pounds of liquefied natural gas charged to the carrierat the field site to provide about 1 pound of natural gas at the marketsite and to compensate for loss by evaporation of natural gas in thetransportation stage, said refrigerant being compressed at the marketsite and then being passed into heat exchange relation with liquefiednatural gas, said refrigerant being further cooled in a main stream byreducing its pressure and being split into a product stream and arecycle stream.

9. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant composed principally ofnitrogen, transferring the liquefied natural gas to an insulatedtransport carrier for shipment to the market site, transferringliquefied refrigerant from an insulated transport carrier which is alsoused as a carrier for liquefied natural gas to the field site for use insaid aforementioned step of liquefying natural gas at the field site,said liquid refrigerant being compressed before it is passed in heatexchange relation with the natural gas and after said heat exchangebeing expanded to a lower pressure and lower temperature, said naturalgas being compressed at the field site before it is passed in heatexchange relation with the refrigerant and after said heat exchangebeing expanded to a lower pres- 13 sure, the work provided in theexpansion of the natural gas being used in the compression of therefrigerant and the work provided in the expansion of the refrigerantbeing used in the compression of the natural gas.

' 10. The method of claim 9 in which the refrigerant is passed in heatexchange relation with the natural gas in a multiplicity of passesfromsubstantially the same temperature.

11. The method of claim 9 in which the refrigerant after initialexpansion is passed in heat exchange relation with the natural gas in atleast one additional pass at a reduced temperature obtained byadditional expansion of the gas to a lower pressure.

12L'The method of claim in which the refrigerant after initial expansionis passed in heat exchange relation with the natural gas in at least oneadditional pass at a reduced temperature obtained by additionalexpansion of the gas to -a lower pressure.

13. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant composed principally ofnitrogen, transferring the liquefied natural gas to an insulatedtransport carrier for shipment to the market site, transferringliquefied refrigerant from an insulated transport carrier which is alsoused as a carrier for liquefied natural gas to the field site for use insaid aforementioned step of liquefying natural gas at the field site,said refrigerant being passedat the market site in heat exchange withcold natural gas, compressed and cooled by passing in heat exchange withliquefied natural gas, said refrigerant being further cooled in a mainstreamtby expansion to a lower pressure and splitting it into aliquefied product stream and a recycle stream.

, 14. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant composed principally ofnitrogen, transferring the liquefied natural gas to an insulatedtransport carrier for shipment to'the market site, transferringliquefied refrigerant from an insulated transport carrier which is alsoused as a carrier for liquefied natural gas to the field site for use insaid aforementioned step of liquefying natural gas at the field site,said liquefied natural gas being split at the market site into first andsecond streams, said first stream being passed in heat exchange relationwith the refrigerant and subsequently compressed for discharge in anatural gas product stream, said second stream being compressed andpassed in heat exchange relation with the refrigerant, said secondstream being subsequently expanded to a lower pressure and combined inthe natural gas product stream.

15. The method of claim 14 in which the work provided in the expansionof the second stream of natural gas is used in the compression of thefirst stream.

-16. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant composed principally ofnitrogen, transferring the liquefied natural gas to an insulatedtransport carrier for shipment to the market site, transferringliquefied refrigerant from an insulated transport carrier, which is alsoused as a carrier for liquefied natural gas to the field site, for usein said aforementioned step of liquefying natural gas at the field site,said refrigeratnt being passed at the market site in heat exchange withcold natural gas, compressed and cooled by passing in heat exchange withliquefied natural gas, said refrigerant being further cooled in a mainstream by expansion to a lower pressure and splitting it into aliquefied product stream and a recycle stream, said liquefied naturalgas being split at the market site into first and second streams, saidfirst stream being passed in heat exchange relation with the refrigerantand subsequently compressed for discharge in a natural gas productstream, said second stream being compressed and passed in heat exchangerelation with the refrigerant, said second stream being subsequentlyexpanded to a lower pressure and combined in the natural gas productstream.

17. The method of claim 16 in which the work pro vided in the expansionof the refrigerant is used in the compression of the second stream.

18. 'The method of claim 16 in which the work provided in the expansionof the refrigerant is used in the compression of the second stream andthe work provided in the expansion of the second stream of natural gasis used in the compression of the first stream.

19. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant composed principally ofnitrogen, transferring the liquefied natural gas to an insulatedtransport carrier for shipment to the market site, transferringliquefied refrigerant from an insulated transport carrier which is alsoused as a carrier for liquefied natural gas to the field site for use insaid aforementioned step of liquefying natural gas at the field site,said liquid refrigerant being compressed before it is passed in heatexchange relation with the natural gas and after said heat exchangebeing expanded to a lower pressure and lower temperature, said naturalgas being compressed at the field site before it is passed in heatexchange relation with the refrigerant and after said heat exchangebeing expanded to a lower pressure, the work provided in the expansionof the natural gas being used in the compression of the refrigerant andthe work provided in the expansion of the refrigerant being used in thecompression of the natural gas, said refrigerant being passed at themarket site in heat exchange with cold natural gas, compressed andcooled by passing in heat exchange with liquefied natural gas, saidrefrigerant being further cooled in a main stream by expansion to alower pressure and splitting it into a liquefied product stream and arecycle stream.

20. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant composed principally ofnitrogen, transferring the liquefied natural gas to an insulatedtransport carrier for shipment to the market site, transferringliquefied refrigerant from an insulated transport carrier which is alsoused as a carrier for liquefied natural gas to the field site for use insaid aforementioned step of liquefying natural gas at the field site,said liquid refrigerant being compressed before it is passed in heatexchange relation with the natural gas and after said heat exchangebeing expanded to a lower pressure and lower temperature, said naturalgas being compressed at the field site before it is passed in heatexchange relation with the refrigerant and after said heat exchangebeing expanded to a lower pressure, the work provided in the expansionof the natural gas being used in the compression of the refrigerant andthe work provided in the expansion of the refrigerant being used in thecompression of the natural gas, said liquefied natural gas being splitat the market site into first and second streams, said first streambeing passed in heat exchange relation with the refrigerant andsubsequently compressed for discharge in a natural gas product stream,said second stream being compressed and passed in heat exchange relationwith the refrigerant, said second stream being subsequently expanded toa lower pressure and combined in the natural gas product stream.

21. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant composed principally ofnitrogen, transferring the liquefied natural gas to an insulatedtransport carrier for shipment, to the market site, transferringliquefied refrigerant from an insulated transport 15 carrier which isalso used as a carrier for liquefied natural gas to the field site foruse in said aforementioned step of liquefying natural gas at the fieldsite, said liquid refrigerant being compressed before it is passed inheat exchange relation with the natural gas and after said heat exchangebeing expanded to a lower pressure and lower temperature, said naturalgas being compressed at the field site before it is passed in heatexchange relation with the refrigerant and after said heat exchangebeing expanded to a lower pressure, the work provided in the expansionof the natural gas being used in the compression of the refrigerant andthe work provided in the expansion of the refrigerant being used in thecompression of the natural gas, said refrigerant being passed at themarket site in heat exchange with cold natural gas, compressed andcooled by passing in heat exchange with liquefied natural gas, saidrefrigerant being further cooled in a main stream by expansion to alower pressure and splitting it into a liquefied product stream and arecycle stream, said liquefied natural gas being split at the marketsite into first and second streams, said first stream being passed inheat exchange relation with the refrigerant and subsequently compressedfor discharge in a natural gas product stream, said second stream beingcompressed and passed in heat exchange relation with the refrigerant,said second stream being subsequently expanded to a lower pressure andcombined in the natural gas product stream.

22. A method for transportation of natural gas from a field site to amarket site which comprises liquefying natural gas at the field site byheat exchange with a liquid refrigerant composed principally ofnitrogen, transferring the liquefied natural gas to an insulatedtransport carrier for shipment to the market site, transferringliquefied refrigerant from an insulated transport carrier which is alsoused as a carrier for liquefied natural gas to the field site for use insaid aforementioned step of liquefying natural gas at the field site,said liquid refrigerant being compressed before it is passed in heatexchange re- 1 lation with the natural gas and after said heat exchangebeing expanded to a lower pressure and lower temperature, said naturalgas at the field site being passed in heat exchange relation with therefrigerant and after said heat exchange being expanded to a lowerpressure, the work provided in the expansion of the natural gas beingused in the compression of the refrigerant and the work provided in theexpansion of the refrigerant being used in part for the compression ofthe natural gas, said refrigerant being passed at the market site inheat exchange with cold natural gas, compressed and cooled by passing inheat exchange with liquefied natural gas, said refrigerant being furthercooled in a main stream by expansion to a lower pressure and splittingit into a liquefied product stream and a recycle stream, said liquefiednatural gas being split at the market site into one or more streams,which are passed in heat exchange relation with the refrigerant, one ofsaid streams being compressed and passed in heat exchange relationwiththe refrigerant, and subsequently expanded to a lower pressure andcombined in the natural gas product stream.

23. The method of claim 7 in which said refrigerant is compressed at themarket site and is passed into heat exchange relation with liquefiednatural gas, said refrigerant being further cooled in a main stream byreducing its pressure and being split into a product stream and arecycle stream, said recycle refrigerant stream being compressed andrecombined in the refrigerant stream passed into heat exchange relationwith the liquefied natural gas.

References Cited UNITED STATES PATENTS 2,975,604 3/1961 McMahon 6255 X3,018,632 1/1962 Keith 6255 X 3,034,309 5/1962 Muck 6255 3,302,4162/1967 Proctor et al 62-55 3,324,670 6/1967 Van Kleef 62-55 ROBERT A.OLEARY, Primary Examiner.

